Lighting of incandescent lamps or light bulbs in, for example, automotive applications typically involves a driving a 12V filament. However, developments in the automotive industry have been made to introduce vehicles with 42V electrical systems and dual voltage electrical systems.
Vehicles with dual voltage electrical systems require two batteries having nominal voltages of 14V and 42V (12V and 36V rated batteries respectively). The 12V battery typically has a high amp-hour rating and is used to provide energy to 14V loads such as lighting circuits, which are difficult to implement at higher voltages, and other applications for example driving 12V resistive loads, like small heaters, and 12V inductive loads, like motors, relays, solenoids. The 36V battery typically has a high cranking current capability and is coupled to a 42V generator and higher voltage loads, which may include the engine starter motor. However, compared with single voltage electrical systems, having an additional power rail such as the 12V power rail in the dual-voltage electrical system increases cost, adds additional weight and reduces efficiency in the system.
Vehicles with 42V electrical systems require a 36V rated battery that is coupled to a 42V generator, as in the dual voltage electrical system. Clearly, in 42V electrical systems all electrical systems, including the lighting system, must be driven by the 36V rated battery.
There are currently several lighting arrangements for 42V and dual voltage electrical systems. A first lighting arrangement that is presently not very practical is to drive incandescent lamps with 42V filaments. However, filaments for 42V/36V are too long and fragile. The 42V/36V bulb filaments are relatively thin filaments when compared to filaments in 12V bulbs, for example, for the same power at 42V/36V filament of the same diameter would be 9 times the length, or alternatively ⅓ the diameter of a 12V filament. 43V/36V bulbs are presently unpractical to use because the thin filaments results in a lifetime that would be unacceptably low in the automotive environment.
Another lighting arrangement that has been proposed for 42V and dual voltage electrical systems is a DC—DC converter to step 42V down to 14V/12V. Although this system provides the convenience of 12V bulbs, it is an expensive solution to implement the DC—DC converter and such a system consumes, for example, more than 300 W. For these reasons, using a DC—DC converter is also presently not practical to implement in an automotive environment.
There are also lighting systems that us PWM for driving lamps. Such a system is disclosed in SU909805. PWM systems typically operate at 120 Hz to replicate 50 to 60 Hz AC operation. However, especially where multiple lights are switched on together, very high peak currents, which may rupture the bulb filament, are reached in the wiring of the lighting system resulting in excessive heat and electromagnetic interference. In 42V applications, for example, when a bulb is first switched on a large current surge flows in a 42V/36V PWM system, which may be three times as large as 12V DC system, which may rupture the bulbs filament. For these reasons, such PWM lighting systems are unacceptable in an automotive environment.
Therefore, there is a need in the art for a cost effective and power efficient way to power a bulb with a lower voltage tolerance, for example 12V or 6–8V, from a higher voltage supply, for example 42V/36V as used in automotive systems and/or 24V as used in truck systems, in an environment wherein the PWM mark-space ratio is variable such as for example in a 42V or dual-voltage electrical automotive systems.